Microbes on Earth may be key to identifying life on other planets

CHAMPAIGN, Ill. -- Evidence of life in Martian meteorites or future rock
samples from the Red Planet may be easier to identify thanks to microbes
living in hot springs at Yellowstone National Park.

"The existence of life itself can change the physical and chemical attributes
in an environment of deposition," said Bruce Fouke, a geologist at the
University of Illinois. "By studying the effects of microbial metabolism on
the chemistry of the water and on the way minerals are deposited in Earth
environments, we can better interpret samples from other planets for signs
of life."

For example, various carbonate features -- including tiny, rod-shaped
calcite crystals -- found in the Martian meteorite ALH84001 could have
been formed by either organic or inorganic means. To help interpret whether
such shapes are indicative of life, Fouke has established a systematic model
for the deposition of travertine by actively flowing hot springs at Angel
Terrace at Mammoth Hot Springs.

"Travertine is a crystalline form of calcite that forms where subsurface
waters erupt, cool, de-gas and precipitate calcium-carbonate minerals with
a variety of crystal morphologies and chemical compositions," Fouke said.
"In this setting, we are examining the environmental feedback mechanisms
that exist between water, microbes and the precipitation of travertine."

Mammoth Hot Springs, near the northern boundary of Yellowstone National
Park, is one of the world's largest sites of travertine accumulation. The
travertine deposits at Mammoth Hot Springs are approximately 8,000 years
old, 73 meters thick and cover more than 4 square kilometers.

"Yellowstone is an ideal laboratory because of the high precipitation rates
and the abundance of microbes," Fouke said. "By documenting where we find
certain calcite shapes in the spring system, we can link those shapes with
a particular water flow, chemistry and microbe. With that environmental
context, we can start to decipher the geological record and to reconstruct
ancient environments."

Geochemical evaluation of the spring water and underlying travertine has
suggested that inorganic processes such as carbon dioxide de-gassing,
temperature decreases and possibly evaporation are the primary
environmental controls on travertine mineralogy, Fouke said. "So the
environmental context could be the key to determining whether or not a
particular feature is an entombed microbe."

On Earth, microbes also can be found trapped in fluid inclusions in ancient
calcite crystals. Fouke is working with UI microbiologist Abigail Salyers
to develop techniques to liberate the microbes and isolate, extract,
amplify and sequence their DNA.

"The genetic analysis will provide additional information about the
microbes' metabolism," he said. "We will incorporate this information
into our depositional model to help link the presence of ancient life
with suspect, calcium-carbonate depositional features and chemical
compositions."

Fouke published his findings in the May issue of the Journal of Sedimentary
Research. Funding was provided by NASA, the National Research Council and
the UI Critical Research Initiative.